1. Field of the Invention
This invention relates to method and apparatus for cleaning gas.
In partciular aspects this invention relates to screw conveyors, cyclones, filters, gravel bed filters and cleaning gas streams.
2. Description of the Prior Art
Reference is made to U.S. Pat. Nos. 3,594,991 and 3,917,472 and to U.S. Pat. No. Re. 28396.
In those U.S. specifications is disclosed, inter alia, a gravel bed filter which is cleaned by passing a gas stream through the filter from below (which being counter current to the direction of flow during filtering). Further, the upper surface of a gravel filter bed is disturbed, during the cleaning operation, by a rake which is moved through the filter bed. In one instance air is passed down the inside of the tines of the rake into the filter bed.
Applicant has found that considerable power is required to drive the rake through the filter bed and in particular that the power requirements are at a maximum on commencing movement of the rake at the beginning of a cleaning operation as the rake must accelerate from a stationary condition and at that time the gravel bed tends to be clogged with residue from filetering which will restrict movement of the rake. Later in the cleaning operation when the rake has been accelerated and after a majority of the residue has been removed this is less of a problem but the major difficulty of initial movement of the rake at the commencement of a cleaning operation remains.
In addition to the power requirement required to initiate movement of the rake as compared to the power requirement to move the rake later in the cleaning operation it has also been observed by applicant that damage to the tines of the rake may occur; particularly at initiation of movement of the rake. This damage includes bending or even breaking of the tines. Further, the tines are normally supported on an arm which is in turn supported by a shaft which is supported in bearings and applicant has found that damage to the arm, shaft or bearings can occur. Damage to the shaft or bearings can be particularly serious as it may hamper rotation of the shaft and/or put undue strain on a motor for rotating the shaft.